Examples of a coil apparatus in which a coil is arranged around a core of a stator core may include electrical devices such as motors, electric generators, and transformers. In such a coil apparatus, it is important to improve the space factor of the coil in the core in order to achieve loss reduction and the miniaturization thereof.
Coils using rectangular conductors (hereinafter referred to as rectangular coils) have been known in the art as coils capable of improving space factors in cores. The rectangular coil is referred to also as a flatwise (square-winding) coil, a square (square type) coil, an edgewise coil, etc. In contrast to a coil formed by winding a round conductive wire having a generally round cross section perpendicular to the longitudinal direction thereof, the rectangular coil refers to a coil formed by winding a rectangular conductor having a rectangular cross section perpendicular to the longitudinal direction thereof.
A method of winding an elongated rectangular conductive wire (square conductive wire) in a generally rectangular shape has been also known in the art as a method of manufacturing a rectangular coal (see Patent Literature 1, for example). A method of making a helical structure by repeatedly performing stacking rectangular conductors (flat conductive materials), each having a length corresponding to one coil turn, on each other and joining such layers together by welding means with the upper surface of an end edge in the lower layer and the lower surface of a start edge in the upper layer being overlapped with each other has been also known in the art (see Patent Literature 2, for example).
A cold pressure welding method has also been known in the art as a method of connecting conductors.